Outline

1. Outline 2.4.1 Introduction 2.4.2 Pseudo-Noise Sequences 2.4.3 Direct-sequence Spread Spectrum 2.4.4 Frequency-Hop Spread Spectrum 2.4.5 Time-Hop Spread Spectrum 2.4.6 Acquisition and Tracking 2.4.7 Performance in a Jamming Environment 2.4.8 CDMA and Smart Antenna 2.4.9 Summary References Problems 2.4.7

2. Classification of jammers The jamming signals can be classified as (1) barrage noise jammer, (2) partial band jammer, (3) single-tone jammer, (4) multiple-tone jammer, (5) pulsed (on/off) jammer, and (6) repeater jammer. (1) Barrage noise jammer transmits bandlimited white Gaussian noise with one-sided power spectral density of watts /Hz, as shown in Figure (a). (2) Partial band jammer, shown in Figure (b), transmits all the available power in a limited band-width which is smaller than the SS signal bandwidth, where the fraction is denoted by . (3) Single-tone jammer transmits an unmodulated carrier with power J somewhere in the SS signal bandwidth. The one sided-power spectrum of this jamming signal is shown in Figure (c). 2.4.7

3. Classification of jammers 2.4.7

4. Classification of jammers (4) For FH system, a better tone jamming strategy is to use several tones which share the power of the single-tone jammer, which is called a multiple-tone, as shown in Figure (d). (5) Another technique for concentrating the jamming power is to pulse the jammer ‘on’ and ‘off’, which is called pulsed noise jammer, the transmitted waveform is shown in Figure (e). (6) The last type of intention jamming is repeater jamming. A repeater jammer receives the SS signal, distorts it in some well-defined manner, and retransmits the signal at high power. Figure (f) shows the transmitted waveform of the repeater jammer . 2.4.7

5. ECM and ECCM For the jammer to be most effective, the jamming signal must be tailored to the SS system and to the actual received signal power. A jammer which has knowledge of the type of signaling being used and which can adapt to transmit the optimum jamming signal is called a smart jammer. The field of study that includes the design and analysis of jammers and jamming strategies is called electronic counter measures (ECM) . Since one of the purposes of SS systems is to counter specific jamming threats, it is included within the general area of electronic counter counter measures (ECCM). It has been shown that even the most sophisticated jammer can almost completely countered by a combination of spectrum spreading, interleaving, and forward error correction. 2.4.7

6. Performance in AWGN with barrage noise jamming For the barrage noise jammer, it is usually assumed that the jammer power spectrum covers exactly the same frequency range as the SS signal. The effect of the jammer on the system is simply to increase the Gaussian noise level at the output of the receiver downconcerter. For DS/BPSK system, the bit error probability (BER) can be rewritten as where is the energy per bit, P is the power of signal, and R is the bit rate. For FH/BFSK system, the BER can be rewritten as 2.4.7

7. Performance in AWGN with barrage noise jamming These relationships are plotted in Figs. using as the independent variable for various SNRs. It can be observed that processing gain is included in these Figs. through the use of the factor W/R in the independent variable. 2.4.7

8. Performance in partialband jamming (DS/BPSK) Except for the transmission bandwidth, the partial band jammer is the same as the wideband barrage noise jammer. In the analysis that follows, W : communication system bandwidth : jammer transmission bandwidth : bandwidth fraction ( ) : full-band jammer one-sided PSD ( ) Assume the jammer using a very narrowband signal and locating close to center frequency, the error probability for DS/BPSK can be given by [4] 2.4.7

9. Performance in partialband jamming (DS/BPSK) The relationship is plotted in Figure. Note that the jammer gain gains about 3 dB ( ) is quickly lost if the jammer center frequency is not properly selected. 2.4.7

10. Performance in partialband jamming (FH/BFSK) For slow-frequency-hop system, the message error probability is calculated using quasi-static analysis. That is, the error probability is calculated separately for thermal noise and thermal plus jamming noise interference, and two results are averaged. Since the fraction of the band that is jammed is , the average bit error probability of FH/BFSK is This equation is plotted in next page for large signal-to-thermal noise ratio using the familiar as the independent variable and using as a parameter. 2.4.7

11. Performance in partialband jamming (FH/BFSK) Observe in Figure that for any value of there is an optimum value of which maximizes the system BER. The jammer has control of and the smart jammer will be able to adjust dynamically to always maximize system BER. 2.4.7

12. Performance in pulsed noise jamming (DS/BPSK) The pulsed noise jammer degrade performance significantly for a small fraction of the time in order to produce the maximum possible increase in average BER. The pulsed noise jammer is on for a fraction of the time and produces a wideband noise signal with one-sided PSD of . It can be shown that the pulsed jammer will produce the same type of degradation for coherent DS systems that the partial band jammer produced for pure FH systems. By some reasonable assumptions and through a quasi-static analysis, the average BER for DS/BPSK is given by [4] 2.4.7

13. Performance in pulsed noise jamming (DS/BPSK) Observe in Figure that for anylue of there also is an optimum value of which maximizes the system BER. 2.4.7

14. Performance in pulsed noise jamming (DS/BPSK) Calculation of the worst-case could be accomplished by differentiating with respect to and setting the result equal to zero. The derivative of the Q-function is not easily calculated, so an alternative means of calculating the worst case is to use an upper bound for the Q-function. For the special case where thermal noise is negligible relative to jamming noise, we have [4] Taking the first derivative and setting it equal to zero and solving for yields . 2.4.7

15. Performance in pulsed noise jamming (DS/BPSK) The duty factor can be no larger than unity so that is used whenever . Now, we only consider the case, . Substituting the optimum into yields This equation is plotted in the right side. We see that for , the pulsed noise jammer has about 31.5 dB degradition to the communication system. 2.4.7